1. Field of the Invention
The invention relates in general to a retention circuit and method, and more particularly to a read state retention circuit and method.
2. Description of the Related Art
A radio frequency identification (RFID) system transmits identification data by radio waves and accordingly the manager can manage goods in a wireless way. The RFID system consists of a number of RFID tags and readers. When the RFID system is applied in goods management, each of the goods has an RFID tag for storing the corresponding identification data, such as a name of goods, goods source, and purchase date. To search or identify the goods in an identification range is a common operation of the RFID system.
When the reader reads the identification data stored in an RFID tag and identifies that it is not the identification data required by the user, the reader sends out an instruction to set the RFID tag to be a “read state”. When the RFID tag receives RF energy sent out by the reader again within a duration, the RFID tag will respond to the reader that it has been read or will not respond to the reader within a predetermined duration. In this way, the number of RFID tags to be read can be gradually reduced until the required identification data are searched or all the required goods are listed.
However, in the process when the reader continues reading the remaining RFID tags which are not read, if the “read state” of the RFID tags which have been read cannot be maintained, there occurs a serious situation that some RFID tags will be read repeatedly, thus reducing search efficiency. In a serious situation, owing that the search time is too long, the “read state” of a large number of the RFID tags which have been read cannot be maintained and these RFID tags are read repeatedly, which causes the required identification data cannot be searched.
Therefore, to ensure the “read state” of the RFID tags which have been read to be maintained during the search time of the reader is an essential subject.